1. Field of the Invention
This invention relates to methods and apparatus correcting the colour of a digital colour image.
2. Description of Related Art
Colour correction of digital colour images is required in a number of image processing contexts.
One important environment is in consumer digital imaging. It is common to produce consumer colour sensors by introducing an alternating pattern of colour filters onto the array of individual sensor elements. This is often referred to as a colour mosaic and a commonly used variant is the RGB Bayer pattern, which has alternating rows of green/red and blue/green pixels (thus having twice as many green pixels as red and blue) on a regular grid. A process known as demosaicing is carried out to generate a full RGB image from an image captured with such a sensor.
It is very difficult to construct color filters for sensors which exactly match the spectral characteristics of our eyes or which exactly match the primary colours used in computers to represent or display images for this reason it is necessary for the captured images to be processed to correct the sensed colours to the desired colour system. In addition, there may well be more noise in one channel than another, and it is desirable to avoid mixing noise from a noisy channel (typically blue) into a less noisy channel (typically green). The amount computation required may he a significant issue or restraint, particularly in a consumer system with limited processing power.
These issues apply to other contexts in which colour correction is required, such as for images generated by imaging systems having sensor elements that have multiple colour receptors (stacked vertically) at each pixel location, three CCD sensors (one for each colour plane), by flatbed colour scanners, or by other imaging systems in which the colour image is formed from sets of separate, registered RGB images. Certain of these issues may apply to other contexts also such as printing where colour correction is used to map from a standard RGB colour space to the RGB space of the printer (prior to the final transformation to the physical CMYK space of the printer).